U.S. Pat. No. 3,937,792 discloses a method of manufacturing silicon nitride powder. The process includes a step of controlling the exothermic reaction between silicon and nitrogen. The control is made by diluting the nitrogen nitriding gas with an inert gas, specifically, argon. During the nitriding process, if the exothermic reaction is too great, argon is bled into the reaction chamber in order to slow down the reaction. The patentees also propose that the chamber be totally evacuated to reduce the exotherm. This patent does not, in any manner, disclose the use of helium gas as a constituent of the gases used during the nitriding of silicon.
U.S. Pat. No. 4,022,872 deals with the manufacture of finely divided refractory powders using helical flow of hot plasma gases. The patent indicates that hydrogen is the preferred plasma gas, but also indicates that argon, neon and helium are also suitable plasma gases. This patent was cited during a novelty study on this subject matter apparently for the reason that argon, neon and helium, at least for the purpose of plasma gases, were the equivalent of helium. However, as will be demonstrated subsequently in this specification, helium and argon do not affect a nitriding system in the same manner. There are substantial differences between a nitriding system in which helium is used and a nitriding system in which argon is used.
In reports entitled "Nitridation of High-Purity Silicon" contained in both the Journal of Material Sciences, Vol. 10 (1975), page 1242, and in the Journal of American Ceramic Society, Vol. 59, No. 7-8, 1976, page 258, Messrs. A. Atkinson et al reported that the lower the nitriding gas pressure in a nitriding operation, the greater the mass of nitride eventually formed. These articles, however, do not mention the use of a helium addition to a nitriding atmosphere in such a manner that the pressure of the nitriding gas is reduced continuously over the period of the nitriding operation. This factor will also be discussed in greater detail hereinbelow.